This invention relates to the production of phosphorus by the electric furnace method. In particular this invention relates to a process in which superatmospheric reactors are operated at elevated temperatures to render non-hazardous the waste from the electric furnace production of phosphorus.
Elemental phosphorus (P4) is produced commercially by the reduction of phosphate ore in an electric furnace. This process produces two primary waste streams: (a) precipitator slurry, and (b) clarifier underflow.
In the electric furnace process a mixture of calcium phosphate, usually as phosphate shale; carbon, such as coke; and sand (silica, SiO2) flux is charged into an electric furnace and the contents heated to melting temperatures, generally about 3000xc2x0 F. (about 1650xc2x0 C.). The phosphate is reduced by the carbon reductant to elemental phosphorus, which is evolved as a vapor from the molten reaction mass along with carbon monoxide and small amounts of other gases.
Typically two major categories of waste streams are generated by the phosphorus electric furnace process: (a) precipitator slurry, which is produced by the furnace gas cleaning device, often an electrostatic precipitator; and (b) xe2x80x9cphossy water wastexe2x80x9d streams, which are an emulsion or xe2x80x9csludgexe2x80x9d of elemental phosphorus, water and suspended and dissolved solids. These various xe2x80x9cphossy waterxe2x80x9d waste streams can be concentrated into a single clarifier underflow steam. The precipitator slurry and clarifier underflow streams may, if desired, be combined into a single waste stream to be treated.
The gaseous stream from the furnace entrains particles of carbon, phosphate ore, and silica as well as other materials present in the furnace. Some of these particulates are removed from the stream by passing it through a device for separating solids from a gas, such as an electrostatic precipitator. Thus, a precipitator slurry is produced when entrained furnace particles are removed from the electrostatic precipitator and quenched in water. Precipitator slurry typically has a phosphorus content of from about 0.1% to about 1% and a solids content of from about 5% to about 20%.
The phosphorus is recovered from the furnace gases by a phosphorus recovery system. The phosphorus vapor gaseous mixture (mostly, but not completely depleted of insoluble particles) from the precipitator is introduced into a water spray tower. In the spray tower sufficient water is introduced by overhead spray heads to reduce the temperature of the gas below the dew point but above the freezing point (about 44.1xc2x0 C.) of phosphorus. The phosphorus vapor condenses to liquid globules of phosphorus, which fall to a sump at the bottom of the tower and coalesce to a layer of liquid phosphorus.
In general, phosphorus-containing aqueous wastes are formed wherever phosphorus water and suspended solids come into contact to form an emulsion or xe2x80x9csludgexe2x80x9d. Phosphorus is typically stored, handled and transported under water to protect it from atmospheric exposure. These aqueous wastes, or xe2x80x9cphossy waterxe2x80x9d as they are commonly referred to, are produced during storage, shipping, and transfer of phosphorus. Phosphorus-containing aqueous wastes originate at various stages in the operation of the plant, such as from the spray tower sump and as a residue in storage tanks. The waste phosphorus occurs mainly as droplets suspended in the aqueous medium of dissolved and suspended solids; very little phosphorus is dissolved because solubility of phosphorus in water is only 3.3 mg/L. Although the phosphorus concentration can vary over wide limits depending on the point of origin, the aqueous waste stream typically contain from about 0.1% to about 3%, more typically about 0.2 to about 0.6% phosphorus.
The phosphorus-containing aqueous wastes from phossy water are processed in a clarifier. Clarifier underflow, a slurry of water, phosphorus, and dissolved and suspended solids, is removed from the bottom of the clarifier.
Precipitator slurry and clarifier underflow contain the following components:
1. Elemental phosphorus, which is pyrophoric and, under alkaline conditions, can release phosphine (PH3) to the ambient air.
2. Cyanide ion (CNxe2x88x92), complexed cyanide, and insoluble cyanides (collectively xe2x80x9ccyanidesxe2x80x9d), which can leach and/or release hydrogen cyanide (HCN) to the ambient air.
3. Metal phosphides, which can gradually release phosphine to the ambient air.
4. Heavy metals, specifically the twelve metals identified in the RCRA Universal Treatment Standards (UTS), incorporated herein by reference: arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se), silver (Ag), antimony (Sb), beryllium (Be), nickel (Ni), and thallium (TI). xe2x80x9cMetalxe2x80x9d refers to the element in either the elemental form or in an form.
The primary components of the precipitator slurry waste stream that require treatment are cyanides, phosphides, and heavy metals. The slurry also contains metals sulfides and selenides, which, if oxidized during waste processing, can cause the associated metals to solubilize. Also, once the selenate anion (SeO4xe2x88x922) has been formed, it is very difficult to remove from solution. The major component of the clarifier underflow waste stream that requires treatment is phosphorus. However, cyanides, phosphides and heavy metals can also be present in the clarifier underflow and, conversely, phosphorus can also be present in the precipitator slurry. Additionally, the stabilized material should not fail EPA SW-846 Method 1030, incorporated herein by reference, and the concentration of phosphine and of hydrogen cyanide above the stabilized material prior to being sent for disposal should not be greater than 0.3 ppm and 10 ppm, respectively.
Various methods have been developed for treating these streams. Munday, U.S. Pat. No. 5,368,741, for example, discloses a process for the disposal of waste generated in the electric furnace manufacture of elemental phosphorus. The process comprises: (a) treating the mixture with lime, whereby the phosphorus is converted in part into salts of phosphorus acids and in part to phosphine while the heavy metal containing particles are rendered non-leachable; (b) collecting the phosphine from (a); and (c) removing the water from the treated aqueous mixture of (a) to produce clarified water and non-leachable solids having a phosphorus content below burning levels. After the reaction is completed, the mixture is dewatered to produce concentrated solids, which are non-leachable when placed in a landfill. However, this process, which is carried out at under relatively mild conditions, does not address the problem of destruction of cyanides and phosphides.
The invention is a process for treating waste slurry generated in the electric furnace production of phosphorus in which a superatmospheric reactor is operated at an elevated temperature. The process comprises the steps of:
a) heating a mixture of waste slurry and alkali, the waste slurry comprising one or more materials selected from the group consisting of UTS metals, cyanides, elemental phosphorus, and phosphides to produce a treated mixture;
b) cooling the treated mixture and separating the gases formed;
c) treating the gases to remove phosphine;
d) if necessary, adjusting the pH of the treated mixture to at least 11; and
e) filtering the treated mixture to produce a wet cake and a filtrate.
In one embodiment of the invention step a) is carried out in the absence of oxygen. In a second embodiment of the invention step a) is carried out in the presence of oxygen. In a third embodiment of the invention step a) is carried out in the absence of oxygen and the mixture is heated a second time in the presence of oxygen.
The process transforms the wastes from the phosphorus electric furnace process into non-hazardous solid, liquid, and gaseous wastes that meet environmental regulatory requirements including limits associated with phosphorus, cyanide, phosphine and cyanide flux, and TCLP metals, and the filtrate for UTS metals and cyanide. The filtrate may require further treatment to meet UTS requirements for soluble cyanide. However, methods for the destruction of soluble cyanides are known in the art.
The invention simultaneously meets all the environmental requirements. Unlike previous processes for treating phosphorus electric furnace waste, this process also addresses cyanide and phosphide removal. Destruction is one way to remove cyanide and/or phosphine from the process. Typically, and preferably, cyanide and/or phosphine are removed by destruction. Phosphine can be removed by, for example, combustion.
In addition, cadmium, antimony, arsenic, and thallium concentrations in the filtrate (principally in the wet air oxidation modification) are controlled by controlling the pH of the filtrate. The process (especially the wet air oxidation modification) also avoids solubilization of selenium as the selenate anion. Also the process insures that the wet cake passes both the TCLP for cadmium and the limit for cyanides.